The principle of limited cell division potential of somatic cells in vitro as demonstrated by Hayflick and others is well established. In order to overcome this limitation, somatic cell hybridization is routinely used to produce immortalized cell lines for the production of proteins. As will be appreciated by those skilled in the art, somatic cell hybridization involves the fusion of a somatic cell having limited division potential with an immortal tumor cell to produce an immortal hybridoma. In this manner, a continuous cell line can be established from which the desired protein is collected.
In a number of fields, researchers utilize somatic cell cultures derived from normal tissues in order to study the mechanisms underlying intercellular interaction and cellular response to various stimuli. These include such diverse pursuits as evaluating the carcinogenicity of selected agents, determining the activities of various hormones, monitoring the reactions of chemotherapeutic agents, and in general studying the metabolic characteristics of a given cell type. However, the phenomenon of limited cell division of normal cells complicates these efforts and often prevents long-term evaluation of cell sensitivity and induced expression.
More specifically, the study by oncologists of neoplastic transformation of epithelial cells has been severely limited by the relatively limited in vitro population longevity. This has led to the use of human fibroblast cultures which have greater in vitro longevity as the accepted model for transformation studies. However, it has been noted that the analogy drawn between these two distinct cell-types is tenuous at best and that the lack of a true long-term epithelial model has hindered cancer research. This is despite the fact that neoplasms of epithelial origin are the most prevalent type of cancer in humans.
In particular, it is known that conventional human mammary epithelial cells have at most a limited cell division potential of from about 30 to 64 doublings. As disclosed in "A Simplified Method For Passage and Long-Term Growth of Human Mammary Epithelial Cells," In Vitro Cellular and Developmental Biology, Vol 22, No. 1, January, 1986, which is incorporated herein by reference, the inventors of the present invention discovered a method of culturing non-neoplastic human mammary epithelial cells which extends the population longevity of these cells beyond the previously reported limit of 13 doublings to more than 50 generations. Although Hammond and coworkers have reported that doublings in excess of 13 can be achieved by supplementing the culture media with bovine pituitary extract, this method required multiple enzymatic transfers and requires high ionic calcium (Ca.sup.++) concentrations. In the aforementioned work of the present inventors, longevity was achieved by reducing the Ca.sup.++ concentration of the media which in turn reduced an inhibition effect in which glucocorticoides induced terminal differentiation. A significant observation which was made in these studies was that conventional human mammary epithelial cells in culture media greater than 0.06 mM ionic calcium underwent terminal differentiation after only three or four divisions in primary culture. It is believed that there may exist epithelial cell lines developed by others which have extended division potential, although they are not the cell lines of the present invention.
It would therefore be desirable to provide a non-neoplastic epithelial cell line which could be cultured indefinitely to permit long-term evaluation of suspected reactive agents. It would also be desirable to provide such a cell line which produces a normal complement of proteins characteristic of normal epithelial cells. It would be further desirable to provide a method by which epithelial cell sensitivity to suspected reactive agents and cellular expression thereby induced could be studied on a long-term basis. The present invention satisfies these goals by providing non-neoplastic human mammary epithelial cells for use in cell culture studies.